There are many environmental effects that will effect the accuracy of your estimates. If you are comparing to actuals, be sure to take these things into account. PMBoK refers to these elements as "Enterprise Environmental Factors". Some examples are:
Specific training in tool or technique use. If this is immediate and focused, the effect can be massive.
General Training. Done properly, this improves motivation, if nothing else.
Relocations of efforts or personnel
Management Shakeups
Effective resources being removed from your job due to higher priorities elsewhere.
Buck up. This is an unfortunate, but common occurrence. In fact, if you are doing well, you may be rewarded by having your best people reassigned, because you no longer need them. A functional managers job is to make sure that all projects are properly equally covered to the best of his resource capability. The efforts with more difficult problems will get more attention and better resources. This means that the ones that are doing well may be raided.
Tool upgrades/problems. A tool upgrade, and training, before the job, will speed you up. That is why you bought it. A tool upgrade right before the job is iffy, and may slow you down, due to immaturity of the tool. Using a new feature of tool may reveal that the tool is actually a drag.
General up or down corporate news.
A positive leader being inserted into the mix can improve efficiency, and a downer person can really hurt things. Was one of these part of your actuals? Know what your actuals are based on.
I attended a week long management seminar, during which groups of us ran a small company in simulation for a "year". What we discovered was that one individual - CANDY - improved the efficiency of everyone she worked around. We also discovered another person, TRANE, that would bring a whole job down. I think one person we assigned to help TRANE ended up resigning.
This really put things into perspective. If you discover someone like CANDY, and I have known a few, put her in an environment where she can do the most good. TRANE did good work - but just had to work by himself.